1. Field of the Invention
The present invention relates to a piston made of aluminum cast alloy and a method of manufacturing the same.
2. Discussion of the Background
In an internal combustion engine such as an engine of automobile or the like, a piston is essential as a part constituting the internal combustion engine, conventionally, the piston has been manufactured by employing an aluminum cast alloy. For the aluminum cast alloy, a variety of alloys have been proposed and improved.
For example, in JP 1996-104937 Unexamined Patent Publication (Kokai), which has been entitled as “Aluminum alloy for an internal combustion engine excellent in strength at a high temperature and method of manufacturing the same”, an alloy containing Cu (Copper): 3-7 mass %, Si (Silicon): 8-13 mass %, Mg (Magnesium): 0.3-1 mass %, Fe (Iron): 0.1-1.0 mass %, Ti (Titanium): 0.01-0.3 mass %, P (Phosphorus): 0.001-0.01 mass %, Ca (Calcium): 0.0001-0.01 mass %, and further containing Ni (Nickel): 0.2-2.5 mass % if it is necessary, and in which P (Phosphorus)/Ca (Calcium) is adjusted in the range from 0.5 to 50 in weight ratio has been published.
The alloy indicated in JP 1996-104937 is characterized in that the foregoing alloy maintains an excellent abrasive resistance comparing to the conventional alloys, and further, the strength at a high temperature has been improved. However, the thermo-mechanical fatigue property has not been considered at all, and there is a problem that the thermo-mechanical fatigue resistance of the above alloy is low. Furthermore, there is a problem that pores tend to be easily generated and the fatigue property is widely dispersed.
Moreover, in the above-described alloy, the strength at a high temperature is enhanced to some extent by increasing the components for enhancing the heat resistance such as Cu (Copper), Ni (Nickel) and the like, however, if the amount of addition of these is increased, there rises a problem that the ductility is lowered, thereby the fatigue strength and thermo-mechanical fatigue property is lowered. Moreover, if the amount of Cu (Copper) is high, the final solidification portion in which Cu (Copper) compound is crystallized dots within the material, pores are generated at that portion by solidification shrinkage.
In this way, practical fatigue properties such as thermo-mechanical fatigue property and so on, which are required for the piston top face section, cannot be enhanced at all only by the conventional methods in which heat resistance components are increased.